In January 1998, the European Telecommunications Standards Institute (ETSI) selected the basic technology for the Universal Mobile Telecommunications System (UMTS) (see ETSI, “The ETSI UMTS Land Radio Access (UTRA) ITU-R RTT Candidate Submission”, June 1998). The main radio interface proposed was the Wideband Code Multiple Division Access (WCDMA) protocol, the characteristics of which offer the opportunity to fully satisfy the requirements of third generation (3G) mobile telephony. Due to the high data transmission rate and to the increasingly demanding requirements for quality of service (QoS) in 3G, the development is required of new planning strategies. Among them, that which is probably the greatest object of study is the system of power control, in particular that of the procedure employed to implement the outer loop of said system.
The aforementioned system of power control is described below in general terms, because the functionality of the outer loop, for which this invention proposes a method, is a consequence of other components of the system.
The system of power control in WCMDA-based cellular networks, is necessary since it concerns a technology limited by interference, because all the users share the same frequency spectrum and their codes are not totally orthogonal (see Holma & Toskala: “WCDMA for UMTS, Radio Access for Third Generation Mobile Communications”, John Wiley & Sons).
The ultimate goal of the power control system in WCDMA is to attain the required quality of service in a particular connection, downlink from the base station to the mobile terminal or terminal unit, or, uplink from the mobile terminal to the base station, with a minimum level of transmitted power (this aspect is precisely that on which the invention is centred).
The main objectives of the system of power control in WCDMA networks are:
                Cancellation of the near-far effect: in the event of all the mobile stations transmitting the same power without taking into account the distance or the fading to the base station, the mobiles nearest the same would signify substantial interference for the most remote terminals.        Protection against deep fading.        Minimization of the interference in the network with the ensuing improvement in capacity.        Enhanced duration of the battery of the mobile stations.        
A system of power control for WCMDA is implemented overall by means of three distinct procedures:                By open loop: during the random access process when setting up a connection, the base/mobile station estimates the loss of power in the uplink/downlink connection and in terms thereof adjusts its transmission power.        By closed or inner loop: also termed fast power control (1500 Hz) which consists of the following three steps:            1. The corresponding receiving terminal (the base station or the mobile unit) compares the value of the received desired signal to interference ratio (SIRrec) with the target desired signal to interference ratio (SIRtarget) which depends on the quality of service required for that specific connection and which is fixed by the outer loop procedure explained below.    2. The same receiving terminal sends power control bits indicating that the transmission power should be increased (if SIRrec<SIRtarget) or decreased (if SIRrec>SIRtarget) in a certain value (usually 1 dB).    3. The transmitting unit (base station or mobile) increases or decreases its power in the previously fixed amount.            By outer loop (OLPC, Outer Loop Power Control): this is much slower than the closed loop (10-100 Hz) and establishes the target desired signal to interference ratio (SIRtarget) which causes a predetermined quality objective to be maintained. A criterion or a measurement of the quality of a connection is the frame error rate (FER) or equivalently the block error rate (BLER), which is a function of the desired signal to interference ratio (SIRrec). Since the inner loop helps to maintain the desired signal to interference ratio (SIRrec) near the target (SIRtarget), the block error rate (BLER) is, ultimately, determined by this target value. Thus, to attain a quality of service in a determined fading environment, the target (SIRtarget) needs to be adjusted to the value appropriate for that environment.        
Unfortunately, a target (SIRtarget) does not exist which can attain the block error rate (BLER) required for all the fading environments in the wireless communication channel. For this reason, the dynamic adjustment of this target desired signal to interference ratio (SIRtarget) is today an object of study and mechanisms have been described to adjust said ratio conveniently.
The commonly accepted design for outer loop power control (OLPC) is that based on the target block error rate (BLERtarget) and termed “BLER-Based OLPC”, which measures this metric and changes the target desired signal to interference ratio (SIRtarget) in consequence, depending on whether the target block error rate (BLERtarget) is above or below the desired threshold (see Sampath A, Kumar P S & Holtzman J M (1997), “On setting reverse link target SIR in a CDMA system”, Proceedings of the IEEE Vehicular Technology Conference, Phoenix, Ariz., pp 929-933.). The drawback is that, bearing in mind that the technique of measuring the block error rate (BLER) is quite slow, especially for high quality services, the features of these systems are greatly impaired in dynamic environments with fading characteristics changing in very short intervals of time (see Holma H., “WCDMA for UMTS”, John Wiley & Sons, Ltd., 2002). The aforementioned slowness for the services that require a low block error rate (BLER) (for example: 0.1%) is due to the “BLER-based OLPC” method being based on counting the errors by means of the Cyclic Redundancy Code (CRC), which implies an excessively high number of data blocks to arrive at a precise estimate of the block error rate (BLER).
The most serious problem is that which arises when a favourable change occurs in the propagation conditions in which event the “BLER-based OLPC” method reacts very slowly, causing the target desired signal to interference ratio (SIRtarget) fixed by said outer loop power control method to be greater than that necessary for a long period of time, with the consequent increase in interference and, therefore, the loss of system capacity.
Much investigation has been applied aimed at resolving the slow convergence of the power control method which, as has been explained, occurs in the “BLER-Based OLPC”. One of the options most employed as a possible solution consists in carrying out modifications to the size of the adjusting steps for the target desired signal to interference ratio (SIRtarget) which is imposed by the cited BLER-based OLPC method (see again Sampath A, Kumar P S & Holtzman J M (1997), “On setting reverse link target SIR in a CDMA system”, Proc. IEEE Vehicular Technology Conference, Phoenix, Ariz., pp 929-933.). However, that option does not overcome the inherent problem with this type of power control method since it also involves a very high number of data blocks for the precise estimation of the block error rate (BLER). Based on this principle of the quality criterion which obeys the target block error rate (BLERtarget), some methods can be cited which have been object of the following patent applications in the United States: US 2004/0137860, US 2004/0157636 and US 2003/0031135.
Another of the most usual alternatives to overcome the problem of the slow convergence of the BLER-Based OLPC method is the consideration of other metrics (the so-called “soft metrics”), among which are: Bit Error Rate (BER), re-encoded Symbol Error Rate (SER), a metric of re-encoded power, number of decoding iterations, modified metric of Yamamoto and the Euclidean Distance (ED) (see Rege Kiran, “On Link Quality Estimation for 3G Wireless Communication Networks”, in the Proceedings of the IEEE VTS Fall VTC2000. 52nd Vehicular Technology Conference). These metrics have the advantage over the block error rate (BLER) that they can be estimated with much greater speed.
Since the purpose of the OLPC is that of meeting a target of constant block error rate (BLERtarget) and for a moderate change in the block length due to the propagation conditions of the channel, a practically fixed ratio is established between the block error rate (BLER) and the aforementioned “soft metric” parameters, with which it is possible to find the target block error rate (BLERtarget) based on an estimate of any one of said metrics. By way of example, mention can be made of some designs of methods based on these metrics which have been object of the following patents: U.S. Pat. No. 6,434,124 and U.S. Pat. No. 6,763,244.
Nevertheless, the drawback of the outer loop power control based on such metrics arises when a change in the propagation conditions of the channel substantially affects the block length. In this situation, the correlation between the block error rate (BLER) and the metrics considered as “soft metrics” are no longer fixed and therefore a constant block error rate (BLERtarget) is not obtained (see Avidor, Dan, “Estimating the Block Error Rate at the Output of the Frame Selector in the UMTS System”, in Proceedings of the Wireless Networks and Emerging Technologies (WNET 2002), Wireless and Optical Communications (WOC 2002), July 2002, Banff, Alberta, Canada.).
On the other hand, Jonas Blom, Fredrik Gunnarson and Fedrik Gustafsson in their patent application U.S. Pat. No. 6,449,462, establish a method to control the target desired signal to interference ratio (SIRtarget) also based on measuring the block error rate (BLER), but together with the calculation of some determined representative parameters of the different conditions of the radiofrequency channel and of the statistical distribution of the interfering signals. The method is based on the determination of a quality function defined as the errored frame probability conditioned by the aforementioned parameters. Although this strategy implies gains in capacity of the order of 30%, the process for obtaining said quality function imposes a delay which impairs the benefits of this type of model. Separately, in the article by the same authors in which the invention is described in more technical detail: “Estimation and Outer Loop Power Control in Cellular Radio Systems” presented at ACM Wireless Networks, it is stated that the system can be degraded due to fading in the radiofrequency channel.
The applicant of the present patent, Álvaro López Medrano in Spanish patent application ES 200202947 (see also the articles by Álvaro López-Medrano: “Optimal SIR target determination for Outer loop Control in the W-CDMA System”, Proceedings of the IEEE Vehicular Technology Conference (VTC) Fall 2003, 6-9 Oct. 2003, Orlando (USA) and “Optimal SIR target determination for Outer loop Control in the W-CDMA System: Inverse SIR Cumulative Distribution Function computation throughout the Newton-Raphson Method”, Proceedings of the 12th IST Summit on Mobile and Wireless Communications (Volume II), pp. 732-736, 15-18 Jun., 2003, Aveiro, Portugal) proposes an outer loop of the power control system in 3G systems based on a quality criterion different to that of the target block error rate (BLERtarget). This quality criterion on which the method described in ES 200202947 is based, is the outage probability (Poutage), with which the aforesaid inherent low speed of convergence of the BLER-based OPLC method is avoided.
As is explained in ES 200202947, the outage probability (Poutage) constitutes another habitually applied quality parameter in cellular infrastructures, which is established previously, during the planning phase of the communications network, in terms of the class of service covered by the communication link, the characteristics of the cells and, inside each cell, the characteristics of the service area. Based on this outage probability (Poutage), it is proposed in the aforementioned patent application to determine the fading margin (M(Sii) (dB)) corresponding to the desired signal to interference ratio and, therefore, the target desired signal to interference ratio (SIRtarget) for a quality of service criterion given by the outage probability (Poutage) and some characteristic statistical moments of the radiofrequency channel under consideration.
The explanation given in the preceding paragraph is expressible as a mathematical problem first proposed by S. Kandukuri and S. Boyd (in IEEE Transactions on Wireless Communications, vol. 1, no. 1, pp. 46-55, January 2002) and known as “Optimal power control in interference-limited fading wireless channels with outage-probability specifications”, which was resolved by Álvaro López Medrano in his previously cited patent application, by applying the iterative method of Newton-Raphson (see H. R. Schwarz, J. Waldvogel “Numerical Analysis”, John Wiley & Sons) to outer loop power control.
In brief, the outer loop power control method proposed by López Medrano in the previous patent application ES 200202947 is based on the quality criterion of outage probability (Poutage), but a final commitment of an outer loop must be to maintain constant a target block error rate (BLERtarget) which corresponds to a determined service (see the specification documents of the Third Generation Standard 3GPP: TS 25.101, “UE radio transmission and reception (FDD), section 8.8.1” and the TS 25.104, “Base station (BS) radio transmission and reception (FDD), section 8”). Consequently, it is not possible to maintain a constant outage probability (Poutage) for all propagation conditions, as the actual block error rate (BLER) does not remain constant. This is because there is no fixed ratio between the outage probability (Poutage) and the block error rate (BLER), but instead it depends on the propagation conditions in the radio link that are taking place at that moment.
As the fading margin, which is the outcome of the outer loop power control method disclosed in ES 200202947, is a function of such an outage probability (Poutage) among other variables, the dynamic adaptation thereof implies changes in said margin. And in conclusion, the target desired signal to interference ratio (SIRtarget) ought to be adjustable contemplating the changes in the fading margin, to adapt the outer loop power level to whatever propagation conditions, the power to be transmitted being minimum.